A multibladed cooling air fan assembly 10 (which incorporates the present invention) is shown in FIG. 1. Designed for use in a land vehicle, fan assembly 10 induces air flow through a radiator to cool the engine. Fan assembly 10 has a hub 12 and an outer, rotating ring 14 that prevents the passage of recirculating flow from the outlet to the inlet side of the fan. A plurality of blades or airfoils 100 (seven are shown in FIG. 1) extend radially from hub 12 to ring 14.
To improve the operation of fan assembly 10, much attention has focused on the design or shape of the airfoils. High lift and efficiency are required to meet the ever-increasing operational standards for vehicle engine-cooling fan assemblies. There are many different airfoil shapes and slight variations in shape alter the characteristics of the airfoil in one way or another.
Because only slight variations in airfoil design yield large differences in aerodynamic performance, a multitude of different airfoils were developed by approximately 1920. At that time, there was no orderly system of identifying the different airfoils. Those that seemed to prove effective were simply given arbitrary designations such as RAF 6, Gottingen G-398, and Clark Y.
The National Advisory Committee for Aeronautics (NACA), which was the forerunner of NASA, developed an identification system in the late 1920s. NACA's wind tunnel tests showed that the aerodynamic characteristics of airfoils depend primarily upon two shape variables: the thickness form and the mean-line form. NACA then proceeded to identify these characteristics in a numbering system for the airfoils.
The first such airfoils are referred to by the NACA four-digit series. The NACA 2412 airfoil is a typical example. The first number (2 in this case) is the maximum camber in percent (or hundredths) of chord length. The second number, 4, represents the location of the maximum camber point in tenths of chord and the last two numbers, 12, identify the maximum thickness in percent of chord. All characteristics are based on chord length (c) because they are all proportional to the chord. For this airfoil, the maximum camber is 0.02c, the location of maximum camber is 0.4c, and the maximum thickness is 0.12c.
The flat plate 20, shown in FIG. 2a in an air stream 18, is the simplest of airfoils. At zero angle of attack (.alpha.), flat plate 20 produces no lift because it is actually a symmetrical airfoil (it has no camber). At a slightly positive angle of attack, however, flat plate 20 will produce lift, as shown in FIG. 2b. Flat plate 20 is not a very efficient airfoil because it creates a fair amount of drag. The sharp leading edge 22 also promotes stall at a very small angle of attack and, therefore, severely limits the lift-producing ability of flat plate 20. The stall condition is illustrated in FIG. 2c.
For these reasons, airfoils were provided with a curved nose adjacent the leading edge. That modification enables the airfoil to achieve higher angles of attack without stalling. Such an airfoil is efficient, however, only over a small range of angles. Accordingly, the curved nose was filled in so that a wider range of angles of attack was possible. These thicker airfoils displayed greater lifting capability and finally evolved into the shape shown in FIGS. 3a and 3b, recognized as the "typical" or "classic" thicker airfoil 30.
FIG. 3a illustrates the conventional thicker airfoil 30 having a leading edge 32, a trailing edge 34, and substantially parallel surfaces 36 and 38. The chord of thicker airfoil 30 is the straight line (represented by the dimension "c") extending directly across the airfoil from leading edge 32 to trailing edge 34. The camber is the arching curve (represented by the dimension "a") extending along the center or mean line 40 of thicker airfoil 30 from leading edge 32 to trailing edge 34. Camber is measured from a line extending between the leading and trailing edges of the airfoil (i.e., the chord length) and mean line 40 of thicker airfoil 30.
As shown in FIG. 3b, when thicker airfoil 30 contacts a stream of air 18, the air stream engages leading edge 32 and separates into streams 42 and 44. Stream 42 passes along surface 36 while stream 44 passes along surface 38. As is well known, stream 42 travels a greater distance than stream 44, at a higher velocity, with the result that air adjacent to surface 36 is at a lower pressure than air adjacent to surface 38. Consequently, surface 36 is called the "suction side" of thicker airfoil 30 and surface 38 is called the "pressure side" of thicker airfoil 30. The pressure differential creates lift.
Airfoils with the classic profile of thicker airfoil 30 illustrated in FIGS. 3a and 3b have been used in engine-cooling fan assemblies. Such airfoils improved fan efficiency relative to contemporary, competing airfoil profiles. They have been unable, however, to provide the higher lift-to-drag ratios now desired for automotive applications. High lift and increased efficiency are needed to meet higher operational standards for vehicle engine-cooling fan assemblies. Accordingly, additional airfoil designs have been developed.
U.S. Pat. No. 5,151,014, assigned to Airflow Research and Manufacturing Corporation (ARMC), discloses an airfoil having a reduced, substantially constant thickness over most of its chord length. Accordingly, the ARMC airfoil 50 (see FIGS. 4a, 4b, and 4c which correspond to FIGS. 2a, 2b, and 3, respectively, in the '014 patent) is lighter than thicker airfoil 30 and, ostensibly, offers increased efficiency. ARMC airfoil 50 has a leading edge 52, a trailing edge 54, and substantially parallel suction surface 56 and pressure surface 58.
Pressure surface 58 has a first sharp corner 60, such that pressure surface 58 diverges or bends towards suction surface 56, thereby creating a thick nose section 62 and a reduced thickness portion 64. The distance between corner 60 and leading edge 52 is between 5% and 10% of the chord length of ARMC airfoil 50. Pressure surface 58 also has a second sharp corner 61 upon termination of straight line portion 59 of pressure surface 58. The dashed line 66 in FIGS. 4a and 4b illustrates the pressure surface of thicker airfoil 30.
FIG. 4b illustrates the flow of air over ARMC airfoil 50. A stream of air 18 intersects ARMC airfoil 50 at leading edge 52 and separates into streams 68 and 70. Stream 68 flows along suction surface 56. Stream 70 may not flow, however, along pressure surface 58. According to the '014 patent, stream 70 will separate from pressure surface 58 at corner 60 and will follow a path similar to the path followed by stream 44 for thicker airfoil 30 shown in FIG. 3b. Therefore, ARMC airfoil 50 appears to have substantially the same flow characteristics as thicker airfoil 30.
To assure that stream 70 separates from pressure surface 58, the angle at which pressure surface 58 diverges at corner 60 must be greater than a threshold angle. If the bend is too gradual, stream 70 will turn at corner 60 and remain close to pressure surface 58--resulting in increased loading and noise. Referring to FIG. 4c, corner 60 bends at an angle .theta. of at least 30.degree.. Angle .theta. is measured between lines tangent to pressure surface 58 on each side of corner 60. Although the air flow disclosed in the .div.014 patent may occur, it is unnecessary for the design of a high-lift, lightweight airfoil.
U.S. Pat. No. 4,692,098, assigned initially to General Motors Corporation, discloses an airfoil shaped for improved pressure recovery. In this design, a discontinuity in the form of a flat, step, scribe mark, cavity, or surface roughness is made on the suction surface 86--rather than on the pressure surface 88--of the discontinuous airfoil 80 of the '098 patent (see FIG. 5 which corresponds to FIG. 4 in the '098 patent). Preferably, a flat 82 transverse to the chord of discontinuous airfoil 80 and adjacent to the airfoil nose 84 is provided on suction surface 86. Flat 82 extends rearward from a sharp edge 94 that is located toward the forward end of the laminar boundary layer region. Flat 82 forms a ramp that makes a 9.degree. angle with a tangent line 96 to the upstream suction surface 86 of discontinuous airfoil 80. Discontinuous airfoil 80 also has a rounded leading edge 90, a trailing edge 92, and a so-called Stratford recovery region that connects flat 82 to trailing edge 92.
Discontinuous airfoil 80 is designed to control the size and location of the laminar separation bubble that forms on suction surface 86 as the airfoil operates in a low-Reynolds-number environment. Airfoils of this type are very effective at reducing the size of the laminar separation bubble and ensuring the re-attachment of flow on suction surface 86. By controlling the separation and re-attachment in this manner, discontinuous airfoil 80 operates at a high lift-to-drag ratio.
Airfoils like discontinuous airfoil 80 have been used for many years in engine-cooling fan assemblies on General Motors vehicles. On an airfoil with a straight planform, a discontinuous airfoil 80 with a flat 82 provides excellent performance across a wide operating range. On the new, backward-curved blades used (for example) in the air conditioning systems without chlorinated fluorocarbons (CFCs), however, discontinuous airfoil 80 is not as effective as an airfoil with a smooth, continuous suction surface.
To overcome the shortcomings of conventional airfoils, including ARMC airfoil 50 and discontinuous airfoil 80, a new airfoil 100 is provided. An object of the present invention is to provide an improved, light-weight airfoil with high lift. Another object is to provide an airfoil having both a smooth, continuous suction surface and a smooth, continuous pressure surface--neither surface having a discontinuity. Still another object is to provide an airfoil that turns an airflow using a highly cambered, thin aft section, while delaying flow separation using a bulbous nose adjacent the leading edge. The required turning is achieved, according to another object of the present invention, with lower mass and higher aerodynamic efficiency than comparable airfoils. An additional object is to provide an improved airfoil that avoids stall at a large range of attack angles; minimizes drag (and, consequently, has a high lift-to-drag ratio); and reduces loading and noise.
A related object is to improve the operational and air-pumping efficiencies of an engine-cooling fan assembly having a plurality of airfoils. Airfoils produce turning of the air stream through the assembly, thereby creating a pressure rise across the assembly. Yet another object of the present invention is to provide an airfoil in an engine-cooling fan assembly that provides high pressure rise across the fan assembly and reduced mass. It is still another object of the present invention to reduce the axial depth of the ring of the assembly. Finally, it is an object of the present invention to provide an airfoil design suitable for the entire range of engine-cooling fan assembly operation, including idle.